Mean warming enhances insect resilience to high temperatures: species-specific evidence from two wheat aphids.

Under climate change, the gradual increase in average temperature is often accompanied by more frequent extreme heat events. However, their biological effects have largely been studied in isolation, overlooking the key interactions between them. Specifically, in short-lived ectotherms, it remains unclear whether mild thermal history across multiple generations alters their thermal performance, thereby influencing their response to subsequent high temperature. Here, we addressed this important issue using a full factorial experiment in which two global aphid species, Sitobion avenae and Rhopalosiphum padi, were exposed to baseline (20 ± 0 °C) and warming regimes (25 ± 0 °C and 25 ± 6 °C) for either short-term (14 days) or prolonged (60 days) durations. We then quantified their high-temperature performance, including acute thermal tolerance (CTmax), life history traits and demographic parameters at 30 °C. Our results indicated that pre-exposure to mean warming increased the capacity of both aphid species to withstand subsequent heat events, as evidenced by improved thermal tolerance and fitness-related traits at high temperatures. Notably, the magnitude and persistence of these responses differed between species. Under prolonged warming, S. avenae showed increases in acute heat tolerance and fitness, whereas R. padi improved fitness despite reduced tolerance, likely due to differences in effective acclimation temperatures. Overall, our study bridges the two interconnected facets of climate warming, providing a more realistic perspective on ectotherm resilience in a warming world.